It is typical for a crankshaft in a 4-cylinder, in-line engine to have five main bearing journals. A cross-section of a portion of such an engine 10 is shown in FIG. 1. Engine 10 has four cylinders 12 in which four pistons 14 are installed. Pistons 12 are coupled to a crankshaft 22 via connecting rods 18 that couple between a wrist pin 16 on piston 14 a conrod bearing journal 28 on crankshaft 22. Connecting rod 18 is coupled to wrist pin 16 by having the wrist pin slide through an opening at one end of connecting rod 18 as well as openings through piston 14. Connecting rod 18 has a bearing cap 20 that secures it to conrod bearing journal 28. A shell bearing is commonly placed between a concave surface of connecting rod 18 and the outer, convex surface of conrod bearing journal 28 (not shown in FIG. 1).
Crankshaft 22 has main bearing journals 23, 24, 25, 26, and 27 such that there is a main bearing journal provided between each adjacent pair of conrod bearing journals 28 and one at each end of engine 10 as well. The block of engine 10 has an upper portion 32 and a lower portion 34, the latter of which is sometimes called a ladder. Main bearing journal supports 35, 36, 37, 38, and 39 are provided in lower portion 34 of the block and main bearing journal supports 135, 136, 137, 138, and 139 are provided in upper portion 32 of the block. Bearing journal supports 35 and 135 support main bearing journal 23 with a shell bearing 30 provided between journal 23 and supports 35, 135. A shell bearing is provided for each main journal bearing 23, 24, 25, 26, and 27.
In FIG. 2, a closer view of a prior art crankshaft 40 shows five main bearing journals 42, 44, 46, 48, and 50 with conrod bearing journals 52, 54, 56, and 58 each between an adjacent pair of main bearing journals. Center lines of main bearing journals 42, 44, 46, 48, and 50 are coincident with an axis of rotation 60 of crankshaft 40, whereas, centerlines of conrod bearing journals 52, 54, 56, and 58 are offset from axis 60.
Reducing the number of main bearing journals is desirable to reduce the rotational friction of the engine. This is known in the prior art, but is generally not in use because of inferior bending, torsional stiffness, durability, and NVH characteristics of such a configuration. An example of such a crankshaft 80 is shown in FIG. 3. Three main bearing journals 82, 84, and 86 are provided with: two conrod bearing journals 92 and 94 between adjacent main bearing journals 82 and 84 and two conrod bearing journals 96 and 98 between adjacent main bearing journals 84 and 86. Elements 100 and 102, which could be machined to form main bearing journals in a crankshaft with five main bearing journals, are simply bridges in crankshaft 80. Bridges 100 and 102 are unmachined such that outer surface is rough. In the embodiment shown in FIG. 3, the surface is slightly curved to avoid any stress risers, i.e., clearly not a bearing surface. It was found through modeling that bridges 100 and 102 have insufficient torsional stiffness and resistance to bending for the design goals of cylinder pressure and forces on the crankshaft, thereby would lead to premature failure.
In FIG. 3, counterweights 110, 112, 114, and 116 are provided on crankshaft 80. Furthermore, webs 120, 122, 124, 126, 128, 130, 132, and 134 are arranged with respect to the journal bearings as indicated in the table below:
GroupingsLeft webJournal bearingRight web1st journal120921222nd journal124941263rd journal128961304th journal13298134
Web 130 is different than webs 120, 122, 124, 126, 128, 132, and 134. Web 130 a machined surface possibly including gear teeth that can be used to drive an oil pump, fuel pump, or other engine accessory. This is a non-limiting example and any of the webs could be machined for an additional purpose.
A need exists for a crankshaft for an inline, 4-cylinder engine that can be supported using only three main bearings to meet the design goals without undue bending, reduced durability, or other operational difficulties. Furthermore, such crankshaft should be comparable in weight and cost, i.e., cannot include exotic materials and/or costly machining processes.